1. Field of the Invention
The present invention relates to an apparatus and method for attenuating acoustic energy in a fluid flow, and more particularly, to apparatus and methods for mitigating the acoustic energy in the flow over a cavity.
2. Description of Related Art
The problems caused by acoustic resonance associated with flow over a cavity have been studied for at least 50 years. The issue arises when fluid flowing over a more or less continuous surface encounters a cavity in the surface, which creates a large amount of acoustic energy. This situation occurs in contexts ranging from landing gear wells and weapons bays in airplanes, to gas transport and piping systems, to sunroofs in automobiles. This type of cavity flow not only creates high noise levels but also generates severe vibrational stresses. Failing to mitigate cavity resonance increases operational noise levels and can lead to fatigue-induced structural damage. In aircraft weapons bays, the pressure waves associated with the acoustic energy can also interfere with accurate delivery of weapons such as missiles and so-called “smart bombs.”
FIG. 1 illustrates schematically the fluid flow over a cavity, the physics of which have been described in various publications. See, for example, Cattafesta, Louis, et al., “Review of Active Control of Flow-Induced Cavity Resonance,” 33rd AIAA Fluid Dynamics Conference, Jun. 23-26, 2003, Orlando, Fla., AIAA 2003-3567 (“the Cattafesta article”), from which FIG. 1 is taken. When flow over a smooth surface SS, such as an aircraft fuselage, encounters a cavity CA (representing, say, a landing gear well), a fluid boundary layer BL with a thickness δ separates at the leading edge LE of the cavity. This creates a shear layer SL between the free-stream flow U∞ above the cavity and the slower flow inside the cavity. The shear layer, which is an unstable, turbulent flow region, reattaches to the surface SS near the cavity trailing edge TE. The region where the shear layer SL impinges at the trailing edge LE plays an important role in the overall acoustic response of the flow regime, in that small disturbances of the shear layer around the trailing edge induce significant mass flux into and out of the cavity. Acoustic waves feed back from the trailing edge TE to the leading edge LE inside the cavity (arrow FB), and outside the cavity as well if U∞ is subsonic, which reinforces the acoustic resonance in the cavity. The interaction of the acoustic waves with the shear layer SL causes vortices VX to form in the shear layer SL. These vortices impact at the trailing edge TE of the cavity and can reinforce the acoustic energy generated by the flow over the cavity. The role of the vortices VX in the flow system is also discussed in Kook, H., et al., “Active Control of Pressure Fluctuations Due to Flow Over Helmholtz Resonators,” Journal of Sound and Vibration, Vol. 255, No. 1 (2002), pp. 61-76 (“the Kook article”).
If the acoustic energy generated by this type of cavity flow has frequencies that resonate with natural resonance frequencies of the cavity, it will increase the amplitude of the acoustic disturbances. All told, the resulting dynamic loads created by the cavity can reach 160 dB or higher. Mathematical models have been developed for these kinds of flows, and many of the available models have shown good agreement with experiments. This has enabled the development of various devices and systems to mitigate the acoustic energy generated by the flow over the cavity.
The Cattafesta article surveys a number of such devices, separating them into active and passive flow control systems. Active control systems input external energy, such as mechanical or electrical energy, into the flow, while passive controls do not rely on external energy sources. Generally, interest has focused on three types of devices: (1) zero-frequency passive, stationary devices, such as spoilers, fences, ramps, etc., (2) low-frequency active devices, such pulsed blowing devices, and (3) high-frequency active devices, including splash jets, powered resonance tubes, and the like. The Cattafesta article also describes passive control systems that extract energy from the flow itself, such as unpowered resonance tubes and cylinders and rods situated in the boundary layer near the cavity leading edge, as well as a number of active control systems using open-loop and closed-loop feedback control. In addition to the devices and systems discussed in the Cattafesta article, devices and systems aimed at reducing sound pressure levels caused by flow over an open cavity are discussed in U.S. Pat. Nos. 5,818,947, 5,699,981, 6,098,925, 6,296,202, 6,446,904, 6,739,554, and 7,213,788, and in the Kook article.
The majority of known devices are intended to disrupt the formation of the shear layer at the cavity leading edge, with the object of breaking the coupling between the acoustic waves and the free shear layer. Drawbacks of known passive devices include a limited operating range, meaning that they are only effective through a relatively narrow range of flow conditions. As for active systems, they require an actuation system, which can be bulky and difficult to implement in flow environments encountered in aircraft applications. They can also require a significant amount of energy to operate and entail a weight penalty, both of which can be important considerations in aircraft applications especially.
In spite of the drawbacks in implementing known systems, many have proven successful in reducing sound pressure levels, and in the process have revealed interesting properties of cavity resonance mitigation. For one, there are particular forcing frequencies for given open-loop control systems (in which the device is forced without regard to the actual flow in the cavity) that can lead to significant sound reduction. Other systems have shown that actuation amplitude above a certain magnitude does not significantly affect the flow. For example, a system employing a vibrating cylinder ahead of the cavity generally does not reduce sound pressure levels at magnitudes of vibration greater than about 5% of the cylinder diameter.
One known type of active resonance mitigation system uses flaps at the leading edge of the cavity. Two examples of such systems are described in the Kook article and in U.S. Pat. No. 5,818,947. These can be open-loop or closed-loop systems, but they employ actuators to move the flaps. Since these are active devices, they require actuators that can move the flaps through the required range of motion (amplitude), and such actuators carry significant, usually unacceptable, weight and power penalties. The same problem is encountered with pulsed jet devices, namely that generating sufficient mass fluxes to affect the flow in the cavity generally requires an unacceptable level of external power.